Implement REST API v1 — event log/audit trail endpoint

[treblereel]

Context

Part of epic #1 — Build production-ready REST microservice with k8s deployment on top of casehub-engine. This exposes the immutable event log for observability, debugging, and compliance.

What

Implement event log query endpoint:

• GET /api/v1/cases/{caseId}/events — retrieve event log/audit trail for case instance

Acceptance Criteria

• Returns chronological list of all events (worker executions, state transitions, signals received)
• Each event includes: timestamp, event type, actor/worker name, context diff, trace ID
• Supports pagination (offset/limit or cursor-based)
• Supports filtering by event type (e.g., ?type=WORKER_EXECUTION_COMPLETED)
• Returns 404 if caseId doesn't exist
• Error responses follow RFC 7807 format
• Integration tests verify events appear after worker execution, signal processing, state changes

Notes

• Use casehub-engine's event log mechanism for retrieving case events
• Consider including context diffs from ContextDiffStrategy SPI (casehubio/engine #55)
• Event log should be immutable and append-only
• File paths: casehub-engine's event handling, CaseLifecycleEvent, event storage